A new method has been developed for imaging the fine structure of human tissues called a Scanning Acoustic Microprobe. The system detects, on a scale of a fraction of a millimeter resolution, the isotropic scattering strength, directive scattering strength, directivity, and relative attenuation of tissues at each point in the image plane. By computer graphics an image is reconstructed using these parameters. The method will be used initially for differential diagnosis of breast tumors and for following the course of therapy. The data acquisition system, data processing and pattern recognition system, and the reconstruction system are all based on original and proven concepts for pulse-echo ultrasonic tissue characterization and imaging. The Microprobe should have general applicability to diagnosis in other organs of the body, as well as the breast.